#box {
	width: 1920px;
	height: 1080px;
	background: url(../images/index-bg.jpg) center center no-repeat;
	position: relative;
}

#btn {
	display: block;
	width: 200px;
	height: 185px;
	background: url(../images/rotate-static.png) center center no-repeat;
	background-size: contain;
	position: absolute;
	left: 457px;
	top: 475px;
	cursor: pointer;
}

#mask {
	position: fixed;
	top: 50%;
	left: 50%;
	margin-left: -300px;
	margin-top: -50px;
	width: 600px;
	height: 100px;
	line-height: 100px;
	background-color: rgba(0, 0, 0, .3);
	color: #fff;
	text-align: center;
	display: none;
	font-size: 26px;
}